I. Field
The present disclosure relates generally to electronics, and more specifically to techniques for generating oscillator signals in a wireless communication device.
II. Background
A wireless communication device (e.g., a cellular phone) may have a transmitter and a receiver to support two-way radio communication with a wireless communication system. For data transmission, the transmitter may upconvert an output baseband signal with one or more transmit local oscillator (LO) signals to obtain an upconverted signal. The transmitter may further filter and amplify the upconverted signal to obtain an output radio frequency (RF) signal and may then transmit this signal via a wireless channel to base stations in the wireless system. For data reception, the receiver may receive signals from base stations and obtain a received RF signal. The receiver may amplify, filter and downconvert the received RF signal with one or more receive LO signals to obtain an input baseband signal. The LO signals may be generated based on oscillator signals, which may be generated by oscillators within the wireless device.
The wireless device typically includes various analog circuits to condition analog signals in the transmitter and receiver. The analog circuits may include amplifiers, mixers, filters, phase-locked loops (PLLs), LO generators, etc. The analog circuits may operate on analog signals with small signal levels. Hence, the analog circuits should be exposed to as little noise as possible in order to preserve signal quality and achieve good performance.
The wireless device also typically includes digital circuitry to digitally process data being transmitted and/or received. The digital circuitry may include processors, memories, controllers, etc., which may operate based on clocks. Digital circuits typically have large signal swings and generate lots of digital noise including spurs. A spur is an undesired signal at a specific frequency or tone and generated within the wireless device. Spurs may be generated by clocks, by mixing between clocks and oscillator signals, etc. The spurs from the digital circuits may have large levels because of the large and sharp signal swings of the digital circuits.
The spurs from the digital circuits may degrade the performance of the analog circuits in various manners. First, the oscillator signals used to generate the LO signals for frequency conversion by the analog circuits may contain the spurs, which may then degrade a desired signal being received or transmitted. Second, the spurs may mix with out-of-band signal components and generate inband noise that may degrade the signal-to-noise ratio (SNR) of the desired signal. Third, the spurs may appear at the receiver inputs and/or transmitter outputs in a frequency band of interest due to substrate or package coupling paths when the analog and digital circuits are integrated in the same integrated circuit (IC) and thereby degrade the SNR of the desired signal.
To mitigate the adverse effects due to spurs, the analog circuits may be isolated from the digital circuits, which may then reduce the coupling of the spurs from the digital circuits to the analog circuits. This isolation may be achieved by (i) implementing the analog and digital circuits on separate printed circuit boards or separate sections of a printed circuit board or (ii) implementing the analog circuits on one or more analog integrated circuit (IC) dies and implementing the digital circuits on one or more digital IC dies. However, it may be difficult to achieve the desired amount of isolation or to even predict the amount of isolation that can be achieved due to limitations of design tools. Furthermore, it may be desirable to integrate the analog and digital circuits (e.g., on the same IC die) in order to reduce size and cost. Thus, techniques that can mitigate the adverse effects of spurs are highly desirable.